In a world-first, the UK is starting a government-backed trial to test the efficacy of mixing and matching coronavirus vaccines.

The vaccines currently being rolled out now require two shots weeks apart.

Participants will get one shot of the AstraZeneca vaccine followed by a dose from Pfizer, or vice versa.

Guidelines in the UK say the vaccines aren't interchangeable but can be mixed if the same kind isn't available for the second dose or if the origin of the first shot is unknown.

“This study will give us greater insight into how we can use vaccines to stay on top of this nasty disease,” said Jonathan Van Tam, the UK's deputy chief medical officer.

He said that given the challenges of immunising millions of people amid a global vaccine shortage, there would be advantages to having data that could support more “flexible” immunisation campaigns.

COVID-19 vaccines all train the body to recognize the coronavirus, mostly the spike protein that coats it. The ones from AstraZeneca and Pfizer use different technologies. AstraZeneca's uses a common cold virus to carry the spike gene into the body. Pfizer's is made by putting a piece of genetic code called mRNA — the instructions for that spike protein — inside a little ball of fat.

The British research is scheduled to run 13 months and will also test different intervals between doses - four weeks and 12 weeks apart. Initial results from the trial should be available by June.

A study published this week on the Russian-made Sputnik V vaccine showed it was about 91% effective in preventing COVID-19. Some immunologists credit the fact the vaccine uses two slightly different shots, made with similar technology to AstraZeneca's.